A push belt for a continuously variable transmission is generally known. Such a push belt usually comprises two bundles of endless bands being shaped like a closed loop, which are also indicated as rings, and which function as carriers of a relatively large number of linking elements or transverse elements. The linking elements are movably arranged along the entire circumference of the bands, wherein they are able to transmit forces which are related to a movement of the push belt during operation.
In the following description of a linking element, the mentioned directions refer to the situation in which the linking element is part of the push belt. A longitudinal direction of the linking element corresponds to a circumferential direction of the push belt. A vertical transverse direction of the linking element corresponds to a radial direction of the push belt. A horizontal transverse direction of the linking element corresponds to a direction perpendicular to both the longitudinal direction and the vertical transverse direction. The indication of any linking element as subsequent linking element or previous linking element with respect to an adjacent linking element is related to a direction of movement of the push belt.
In the horizontal transverse direction, the linking element is on both sides provided with recesses for at least partially receiving the bundles of bands. For the purpose of supporting the bundles of bands, the linking element comprises carrying surfaces. For the purpose of contact between the linking element and pulley sheaves of a continuously variable transmission, the linking element is on both sides provided with pulley sheave contact surfaces, in the horizontal transverse direction, which are divergent in the direction of the carrying surfaces.
In the vertical transverse direction, the linking element comprises successively a basic portion, a neck portion of which the dimensions in the horizontal transverse direction are smaller than those of the basic portion, and a top portion of which the dimensions in the horizontal transverse direction at the location of the connection to the neck portion are larger than those of the neck portion. The basis portion comprises the carrying surfaces and the pulley sheave contact surfaces. At the push belt, the basic portion is located at the side of the inner circumference of the push belt, whereas the top portion is located at the side of the outer circumference of the push belt. An important function of the neck portion is interconnecting the basic portion and the top portion.
The linking element has two main body surfaces, namely a front surface and a back surface, which extend substantially parallel with respect to each other, substantially perpendicular to the longitudinal direction. At least a part of the front surface of the linking element is designed to abut against at least a part of the back surface of a subsequent linking element in the push belt, whereas at least a part of the back surface of the linking element is designed to abut against at least a part of the front surface of a previous linking element in the push belt.
In the push belt, two adjacent linking elements are tiltable with respect to each other about a tilting line, which is usually defined at the front surface of each linking element, and which extends along the entire width of the linking element. Usually, the tilting line is formed as a convex transition region at the front surface, which constitutes an even and round transition of two portions of the front surface, which are oriented at a relatively small angle with respect to each other. An important function of the tilting line is guaranteeing mutual contact between adjacent linking elements which during operation of the push belt are located between the pulley sheaves of a pulley. The tilting line is intended to arrange that the forces which are related to a movement of the push belt are transmitted from any linking element to a subsequent linking element in a controlled manner, accompanied by a surface pressure which is regarded as allowable, and which is among others determined by the width of the linking element, i.e. the length of the tilting line, and by the extent to which the convex transition region forming the tilting line is curved, so that undesirable very high local load of the linking elements when they are located in the mutually tilted position can be prevented, together with breakage of the linking elements.
The linking element is manufactured from basic material being shaped like a sheet by means of a blanking process. In the blanking. process, a cutting member and a supporting member are applied, wherein the cutting member is designed to cut the linking element from the basic material under the influence of a cutting force, and wherein the supporting member is designed to support the linking element by a supporting force during the blanking process. During the blanking process, the cutting member penetrates the basic material under the influence of pressure, wherein a mutual movement of the cut linking element and the basic material is allowed. At that moment, the linking element is clamped between a cutting surface of the cutting member and a supporting surface of the supporting member. In this process, it is a known aspiration to organize the blanking process such that a quality of a side surface of the cut product is obtained, which is as high as possible, of course balanced with respect to the effectiveness of the blanking process, including the cost of the blanking tools. From the die-cutting technique, a large number of process parameters are known, which are influential in this process, like a play between cutting member and mould in which the cutting member moves during a blanking movement, an extent to which the mould is bevelled, the cutting force and the supporting force.
Preferably, during the blanking process, the front surface of the linking element is formed at the side of the supporting member, whereas the back surface is formed at the side of the cutting member. Due to the pressure being prevalent during the blanking process, the shape of the supporting surface is then a determining factor regarding the shape of the front surface of the linking element, whereas the shape of the cutting surface is a determining factor regarding the shape of the back surface of the linking element.
It has appeared in practice, that breakage, in particular fatigue breakage, of the linking elements can occur unexpectedly, while a clear cause can not be indicated. Further, it has appeared that in a large number of the cases the breakage occurs in the basic portion, wherein an end of the line of the break, which is identified as the end where the breakage initiates, is located in the region where the carrying surface is connected to the neck portion. It is an important objective of the present invention to provide for a suitable process for forming a linking element, wherein linking elements having a reduced chance of danger are obtained.